The work proposed in this application is directed toward the correlation of the structure and function of the proteins of the complement system as well as their control. The areas which are under investigation are: 1. The chemical nature of the fragments of the fourth component of complement (C4) deposited on human erythrocytes, lymphocytes and polymorphonuclear cells. 2. The mechanism by which these fragments are present on the cell membrane. 3. Biochemical studies on C4 and the relationship to the Chido and Rogers antigens. 4. Regulation and deregulation of the classical pathway C3 convertase (C4b2a).